1. Field of the Invention
The present invention relates to a display device suitable for use with a television receiver, a display device or the like using a cathode-ray tube to perform a display or the like, for example. More particularly, this invention relates to a display device using a dynamic focusing circuit to cause a control waveform to follow a fluctuation of a load of a high voltage to thereby solve a problem of a defocusing occurred in the case such as a brightness is high or the like.
2. Description of the Related Art
Display devices such as a television receiver, a display device or the like using a cathode-ray tube to perform a display carry out a focusing control in order to properly focus a beam spot of emitted electron beam on a screen or tubular surface of the cathode-ray tube. FIG. 1 of the accompanying drawings is a schematic block diagram showing an example of a display device in which such a focusing control is effected according to the related art.
As shown in FIG. 1, there is applied a predetermined high-tension DC voltage 61 which will be described later on. Also, there is supplied a correction waveform 62, such as a parabolic waveform matched with horizontal and vertical deflections or the like, for dynamically controlling the focusing in response to a shape or the like of a tubular surface or screen of a cathode-ray tube 65 which will be described later on. This correction waveform 62 is amplified by an amplifier 63 to 600 to 800 Vp-p. The correction waveform 62 thus amplified and the above-mentioned DC voltage 61 are added by an adder 64, and an added voltage from the adder 64 is applied to the focusing grid, e.g. fourth grid (G4) of the cathode-ray tube 65.
Thus, the focusing is controlled in order to cause the beam spot of the emitted electron beam, for example, to be properly focused on the screen of the cathode-ray tube. In this case, the focus of the beam spot is dynamically controlled in response to the shape of the screen of the cathode-ray tube 65 or the like. In particular, in a cathode-ray tube having a flat display screen surface, the focusing is excellently controlled over the whole of the display screen surface.
However, such display devices encounter with the problems which follow:
(1) A beam spot is not properly focused at the portion such as a white character or the like in which a brightness increases rapidly; and
(2) A blooming occurs in a white peak such as a window signal or the like.
A study of examined results with respect to such problems reveals that those problems are mainly caused by a waveform response characteristic of a focusing voltage.
Specifically, in the above-mentioned display device, i.e. the display device using a so-called conventional-type horizontal deflection high-voltage circuit, an arrangement shown in FIG. 2, for example, is used to generate the above-mentioned focusing voltage (high-tension DC voltage 61=FV). As shown in FIG. 2, a high-tension voltage (HV) is derived from one end 72 of a high-voltage secondary winding of a flyback transformer 71 which performs a horizontal deflection, for example, and a middle-tension voltage (MV) for generating a focusing voltage is derived from a middle tap 73 of this high-voltage secondary winding.
On the other hand, in the above-mentioned display device, it is intended to increase a peak brightness in order to improve a contrast and a brightness of a displayed image. In that case, in the above-mentioned display device, which employs the above-mentioned conventional-type horizontal deflection high-voltage circuit, it is frequently observed that a high-voltage regulation characteristic is not satisfactory. For example, when a signal such as the above-mentioned window signal whose high-voltage load current is large is inputted, a high-tension voltage (HV) value tends to be lowered considerably.
Then, in this case, if the high-tension voltage (HV) value is fluctuated, then the middle-tension voltage (MV) value for generating the above-mentioned focusing voltage also will be fluctuated. Specifically, if a beam current increases due to the window signal or the like as shown in FIG. 3A, for example, then the high-tension voltage (HV) value will be lowered considerably as shown in FIG. 3B (high-voltage fluctuation). Then, the middle-tension voltage (MV) value that is used to generate the focusing voltage also is fluctuated as shown in FIG. 3C, for example, (focusing fluctuation).
Here, if the fluctuated amount of the high-tension voltage (HV) and the fluctuated amount of the middle-tension voltage (MV) were fluctuated with a certain constant ratio, a phenomenon such as the above-mentioned defocusing would not occur. However, in actual practice, an integral time constant of a resistor R1 and a capacitor Cl exists in the high-tension voltage (HV) derived from one end 72 of the above-mentioned secondary winding, and an integral time constant of a resistor R2 and a capacitor C2 exists in the middle-tension voltage (MV) derived from the middle tap 73 of the secondary winding as shown in FIG. 2.
Then, in this case, comparing these integral time constants, we have:
R1*C1 greater than  greater than R2*C2
As a result, most of the fluctuation of the middle-tension voltage (MV) to which the integral time constant (R2*C2) is applied is absorbed. Therefore, the fluctuation of the focusing voltage (FV) that follows the fluctuation of the high-tension voltage (HV) cannot be obtained so that the beam spot is unavoidably defocused on the focusing screen.
Incidentally, the capacitor C2 that generates the integral time constant of the above-mentioned focusing voltage (FV) is a DC blocking capacitor used when the correction waveform 62 used to dynamically control the above-mentioned focusing is superimposed upon the focusing voltage (FV) by the adder 64 shown in FIG. 1, for example. Then, in this case, since the signal of the correction waveform 62 is low from a standpoint of impedance, it can be considered that the capacitor C2 is grounded equivalently and is served as an integral capacitor.
The problem inherent in the display device according to the related art is that, when the beam current increases due to the window signal or the like, the fluctuation of the focusing voltage following the fluctuation of the high-tension voltage cannot be obtained, thereby resulting in the beam spot being defocused on the focusing screen.
In view of the aforesaid aspect, it is an object of the present invention to provide a display device in which a focusing voltage may be corrected in response to the fluctuation of a detected high-tension voltage after the fluctuation of the high-tension voltage was detected. According to this display device, there may be obtained the fluctuation of the focusing voltage following the fluctuation of the high-tension voltage to thereby remove the defocusing of the beam spot on the focusing screen.
According to an aspect of the present invention, there is provided a display device using a cathode-ray tube to perform a display. This display device comprises a control means for controlling a focusing voltage in response to the position of a beam spot on the tubular surface or screen surface of the cathode-ray tube and a correction means for detecting the fluctuation of the high-tension voltage applied to the cathode-ray tube and correcting a focusing voltage in response to the fluctuation of this detected high-tension voltage.
According to the display device of the present invention, without improving high-voltage regulations, the defocusing of a beam spot and a blooming caused in a white peak such as a window signal or the like may be improved considerably.